President-elect Barack Obama on Jan. 20 will become the most important leader of a species that has exploded in just six generations from a total population of 1 billion (around 1830) to a point today when teenagers alone number 1 billion, a species that is on a path toward more or less 9 billion people by mid-century. In numbers, think roughly of adding two Chinas on top of the one that exists today. Expectations that he will exert planet-scale leadership are high, as indicated in this letter from Nelson Mandela to the next president.

Mr. Obama will of course be mainly focused first on economic renewal and finding a way forward in Iraq and Afghanistan. But inevitably issues related to humanity’s growth spurt — both in numbers and resource demands — will come to the fore. The Times columnist David Brooks wrote an interesting piece the other day, “A Date With Scarcity,” focused on resource and financial limits facing an American generation that came of age expecting ever more, explaining a time when “demands on the nation’s wealth outstrip the supply.” As he projected, “There will be fiercer struggles over scarce resources, starker divisions along factional lines.”

Many students of global population and resource trends say the same constraints and consequences are likely to play out on a planetary scale. Keep in mind, as I said above, that we essentially live on “Planet Teen” right now. Depending on the level of governance and economic opportunity they experience, today’s young people could either become soldiers or students, agitators or innovators, terrorists or teachers.

So what’s a president to do on issues like climate, population, international development and health, particularly in an era of huge deficits and pressing real-time problems? I’d like to send Mr. Obama’s transition team your 10 best proposals, as determined by their ranking by readers using one of Dot Earth’s newest features.

Here are a couple of thoughts to prime the pump:

Defense and Development: The Pentagon has recently made fostering stability in turbulent states a top priority, on a par with maintaining the capacity to wage war. What resources and initiatives in the military might be adjusted (without new money) to advance development? Defense and Energy: The military also has the world’s largest budget for basic research and development, to the tune of some $75 billion a year (we spend about $1 billion a year on all energy research, as I pointed out the other day). Military leaders already have energy efficiency as a top priority (much of the activity on a battlefield is focused on protecting fuel supplies).Climate and Energy: Mr. Obama has already pledged to convene a world energy forum, somewhat akin to recent efforts by the Bush administration to bring together established and emerging powers that are the top energy users and emitters of greenhouse gases. Given the importance of abundant, clean energy supplies, and the clock ticking toward a pivotal climate-treaty conference in December 2009, many experts see this as a prime priority. Some environmental campaigners, led by Bill McKibben of 350.org, have pressed Mr. Obama or a top surrogate to attend the next climate-treaty conference, in Poland in December, even before he takes office. I’ve heard mixed reactions to that idea from seasoned experts on environmental diplomacy. (Our Green Inc. blog has European reaction related to global warming.)Family Planning: The world has focused for years on curbing the scourge of H.I.V. in Africa, but population groups say this effort, while laudable, has raided money for family-planning programs that could cut the risk of infection and also help speed the transition out of poverty in the world’s poorest places. The graph below, from Population Action International, shows the spending trends. Can United States’ international H.I.V. policy and family planning programs be more integrated without big new investments?